The sensitivity of photolithographic systems based on diazonaphthoquinone sensitized novolac resins is limited by the quantum efficiency of the sensitizer to photoproduct conversion. It is too low for practical use in deep UV work because of the very low power that is available from exposure sources in those wavelengths. A dramatic increase in sensitivity in radiation-sensitive systems, often referred as chemical amplification, is obtained when a species produced in the primary photoreaction independently initiates a catalytic secondary reaction, thus increasing the quantum yield to values above one. For example, systems which photochemically produce a strong acid which then cleaves acid labile groups in a secondary reaction are disclosed in U.S. Pat. No. 3,915,706 for positive working polyaldehydes.
Radiation sensitive mixtures which contain, as a binder, a polymer which is soluble in aqueous alkaline media, a compound which forms a strong acid by a photochemical reaction, and another compound which possesses acid cleavable C--C--C bonds and whose solubility in a liquid developer is increased by the action of acid are disclosed in U.S. Pat. No. 4,678,737.
Photosolubilizable compositions which are readily removable by developing solutions in areas exposed to actinic radiation are taught in U.S. Pat. No. 3,779,778. That composition comprises a water-insoluble organic compound containing one or more acid degradable linkages of the general formula ##STR1## wherein Z may be --OAr, among others, and a photolyzable acid progenitor which upon exposure to actinic radiation generates an acidic condition.
U.S. Pat. No. 4,311,782 shows a radiation sensitive mixture which contains an acid forming compound and a polymeric compound having recurrent orthocarboxylic acid ester groupings for the production of positive relief images.
U.S. Pat. No. 4,491,628 shows a deep UV sensitive resist composition which is made by combining a polymer having recurrent acid labile pendant groups, e.g., poly (p-tert-butoxycarbonyloxy-.alpha. -methylstyrene), with a cationic photoinitiator such as a triarylsulfonium hexafluoroantimonate. Such compositions are taught there to be particularly useful and advantageous when used with deep UV light (200-300 nm) because they give very high resolution images with nearly vertical wall angles even in films thicker than 2 microns.
More recently, U.S. Pat. No. 5,037,721 has taught a positive radiation sensitive mixture that exhibits no change in the development time whether the time between irradiation and development is long or short, and provides high structural resolution of the developed resist that is retained during processing steps subsequent to the development. Said mixture comprises a compound which forms an acid under the action of actinic radiation and a particular type of monomeric acid-cleavable compound which contains an acetal group. This patent has a good discussion of the acid-cleavable materials which had been employed already, and it is incorporated herein by reference for that purpose. Neither the '721 patent nor those mentioned therein, however, address the problem of latent image decay, which causes critical dimensions (CD) to change in the time between the exposure and the post exposure bake step, which is performed before development.
Latent image decay is the most critical problem in chemical amplification systems for imaging semiconductors. In a paper presented at the Regional Technical Conference of the Mid-Hudson Section of the Society of Plastics Engineers in October of 1991 by Nalamasu et al. and entitled Effect of Post-Exposure Delay in Positive Acting Chemically Amplified Resists: An Analytical Study, it was said, "For TBSS/PAG resist formulations, it is crucial to post exposure bake the material immediately after exposure in order to achieve acceptable line-width control." TBSS is a modified acronym for poly t-butoxycarbonyloxystyrene-sulfone and PAG is an acronym for photoacid generators. In another paper presented at that conference, and entitled t-BOC Blocked Hydroxyphenyl-Meth-acrylates: On the Way TO Quarter Micron Deep-UV Lithography, Przybilla et al. said that virtually no increase in dose-to-clear and influence on structure quality was observed for delay intervals of up to 20 minutes in a t-BOC system when a sulfonic acid generator such as triphenylsulfonium triflate is used but that a more severe dependence on the time between exposure and post exposure bake was observed when onium salts with complex fluorides such as antimony hexafluoride as the counter ion were used as the acid generator. The Nalamasu et al. paper and the Przybilla et al. paper are each incorporated herein by reference for their presentation of the problem which this invention solves.